The present invention relates to an image forming apparatus using electrophotography such as a printer, a copier or a facsimile machine, and particularly relates to a fixing unit of the image forming apparatus.
A general image forming apparatus such as a printer, a copier, or a facsimile machine that forms a monochrome or color image includes an image forming unit that forms a developer image (i.e., a toner image), and a fixing unit that fixes the developer image to a recording medium by application of heat and pressure.
Recently, there is an increasing demand for a color image forming apparatus. In the color image forming apparatus, developer images of a plurality of colors are printed on the recording medium in an overlapping manner. Therefore, in order to fix the developer images to the recording medium, the fixing unit necessarily applies a large amount of heat and high pressure to the developer image. For this purpose, a conventional fixing unit has a large nip portion formed by a heating member and a pressure member in order to apply heat and pressure to the developer image for a long time period.
For example, the conventional fixing unit includes a fixing roller (as a heating member), a pressure roller and a pressure pad which are pressed against the fixing roller, and a fixing belt wound around the pressure roller and the pressure pad. The fixing belt contacts the fixing roller at two positions, i.e., two nip portions. The fixing unit of this type (i.e., a belt-nip type) is configured to apply heat and pressure to the developer image at two nip portions (i.e., for a long time period) so as to fix the color image to the recording medium. Such a conventional fixing unit is disclosed in, for example, Japanese Laid-open Patent Publication No. 2005-275371.
However, in the conventional fixing unit, wrinkles may be formed on the recording medium when the developer image is fixed to the recording medium.